1. Field of the Invention
The present invention relates to positive temperature coefficient (PTC) thermistors and methods for designing the PTC thermistors. In particular, the present invention relates to a PTC thermistor including a barium titanate semiconductor ceramic and a method for designing such a thermistor.
2. Description of the Related Art
Barium titanate (BaTiO3) semiconductor ceramics are widely used to fabricate PTC thermistors. In order to further expand the applications of the PTC thermistors, ardent attempts are being made to further decrease the resistance of the thermistors. One such attempt other than development of BaTiO3 semiconductor ceramics having lower resistivity is the use of a multilayer PTC thermistor as disclosed in Japanese Unexamined Patent Application Publication No. 2002-43103.
The FIGURE is a cross-sectional view of a PTC thermistor related to the present invention. The FIGURE shows a multilayer PTC thermistor 1.
Referring to the FIGURE, the multilayer PTC thermistor 1 includes an element main body 4 including stacked ceramic layers 2 made of a BaTiO3 semiconductor ceramic exhibiting a positive temperature characteristic of resistance and internal electrodes 3 each disposed at the interface between adjacent ceramic layers 2.
External electrodes 5 are provided at the ends of the element main body 4. The internal electrodes 3 electrically connected to the external electrodes 5 at one end and the internal electrodes 3 electrically connected to the external electrode 5 at the other end are alternately arranged.
In the multilayer PTC thermistor 1 described above, the internal electrodes 3 contain nickel in most cases. This is because nickel is relatively inexpensive and can provide an Ohmic characteristic to the ceramic layers 2.
The external electrodes 5 contain, for example, silver as a conductive component.
The multilayer PTC thermistor 1 is made by the following process.
First, ceramic green sheets, which contain a ceramic material powder for forming a BaTiO3 semiconductor ceramic and which is the material of the ceramic layers 2, are prepared.
A conductive paste containing nickel, which is the material of the internal electrodes 3, is applied by printing or the like on the ceramic green sheets.
The ceramic green sheets with the conductive paste layers are then stacked, and additional ceramic green sheets not having conductive paste layers are disposed at the top and the bottom of the stack for protection. The ceramic green sheets are then press-bonded and, if necessary, cut to predetermined dimensions to prepare a green compact of the element main body 4.
The green compact of the element main body 4 is baked in a reducing atmosphere so that nickel contained in the conductive paste for making the internal electrodes 3 does not undergo oxidation. As a result, the ceramic green sheets and the layers of the conductive pastes sinter to form the ceramic layers 2 and the internal electrodes 3.
A conductive paste containing silver is applied on both ends of the sintered element main body 4 and baked in air to form the external electrodes 5.
This baking step also serves as a re-oxidation step for the sintered element main body 4. This step imparts a thermistor characteristic to the ceramic layers 2.
Thus, the multilayer PTC thermistor 1 is made.
The above-described multilayer PTC thermistor 1 has a lower resistance because the thickness of each ceramic layer 2 is decreased by the use of the multilayer structure for the element main body 4.
However, thickness reduction of the ceramic layers 2 does not necessarily result in a decreased resistivity intended to be achieved in actual cases.
This problem is described in detail with reference to the FIGURE. Suppose that the element main body 4 is 2.0 mm×1.2 mm in a plan view and a room temperature resistance of 2Ω is observed for a laminate including 10 ceramic layers 2 each being about 100 μm in thickness. Theoretically, a room temperature resistance of 0.02Ω should be observed by decreasing the thickness of each ceramic layer 2 to one tenth, i.e., 10 μm, and increasing the number of the ceramic layer 2 to ten-fold, i.e., 100. However, in actual observations, the room temperature resistance is sometimes only about 0.28Ω.
The tendency of the actual resistance to deviate from the calculated value is more notable as the thickness of the ceramic layers 2 is decreased to 18 μm or less. In extreme cases, no decrease in resistance is achieved despite the thickness reduction.